I don't understand some points of the voltage clamp method. In the resting state, Command voltage and membrane potential are both -65mv. If the command voltage increases to 0mv, current is injected into an axon, so membrane potential become over than the threshold and sodium is going to flow into the axon.
I learned that current is the flow of electrons, so if the command voltage is changed into 0mv, electrons flow into an internal membrane. It implies that membrane becomes more negative, but it contradicts the fact that membrane potential becomes 0 mv. Is there anything I misunderstood?


In voltage clamp, you measure current.

If you voltage clamp the cell near it's resting potential, you need little current to hold the cell.

If you want to voltage clamp the cell depolarized or hyperpolarized relative to rest, you need to inject current (positive or negative). How much current you need to inject depends on the membrane properties of the cell, and the leak conductance. Therefore, measuring the current as a function of the held voltage tells you about these membrane properties.

As you depolarize the cell, yes, you will open voltage-gated conductances, but these conductances also often inactivate (sodium channels especially). That said, yes, if you depolarize a cell and channels open or close, you will record those changes in your voltage clamp. However, you will never get an action potential if you have a good clamp. The whole point of voltage clamp is that you hold the voltage constant. Action potentials are positive feedback increases in voltage. If you clamp a cell at -40mV, even if -40mV is normally above threshold in a current-clamped cell, the cell will stay at -40mV. It doesn't really make sense to think about action potential thresholds in voltage clamp, they don't exist because the threshold phenomenon (voltage positive feedback) is blocked by clamping the voltage.

One of the benefits of voltage clamp is that you can (somewhat indirectly) measure the flow of certain ions. If you set the voltage equal to the reversal potential for some ion, you know the current for that ion is exactly zero. This allows you to make inferences about how much current is flowing (and knowing voltage you can calculate conductance) at other voltages.

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  • $\begingroup$ Bryan, I know you know, but could you add a reference to set an example for new users? $\endgroup$ – AliceD Mar 28 '19 at 9:11
  • $\begingroup$ Thank you for a detail answer, but I wonder why voltage become positive if command voltage injects current into the membrane. I know that current is a flow of electrons, so my guess is that the membrane potentials become negative if the current is injected into the axon because electrons flow into the membrane. is there anything wrong in my guess? $\endgroup$ – uninopkn Mar 28 '19 at 10:12
  • $\begingroup$ @uninopkn Current being electrons is just a convention for deciding arbitrarily which way is positive vs negative. There is also an arbitrary decision for considering which direction across a membrane is 'positive'. Current flows in a circuit, so positive one way around is negative the other way around. Voltage clamp can provide either positive or negative current. $\endgroup$ – Bryan Krause Mar 28 '19 at 13:44

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